I6l8 A MANUAL OF ANATOMY 



Mesoderm. 



1. The various connective tissues — e.g., bone, cartilage, dentine, 

 cement, areolar tissue, fibrous tissue, and blood. 



2. Muscular tissue, striated and plain, except the muscular tissue 

 of the sweat-glands, that which constitutes the musculi arrectores 

 pilorum in connection with the hair-follicles, and that of the iris. 



3. The vascular and lymphatic systems, together with their endo- 

 thelial linings. 



4. The serous and synovial membranes, together with their endo- 

 thelial linings, including all bursal sacs. 



5. The kidneys and ureters. 



6. The testes, and their complicated excretory equipments. 



7. The ovaries, Fallopian tubes, uterus, and vagina. 



8. The spleen. 



The mesoderm exists under two forms, called mesothelium and 

 mesenchyme. 



Mesothelium is that form of mesoderm in which the cells are 

 flattened and form a definite epithelial membrane or layer, known 

 as endothelium, there being only a very small amount of inter- 

 cellular substance. It lines serous membranes, as well as the 

 chambers of the heart, the bloodvessels, and the lymphatic vessels. 



Mesenchyme is that form of mesoderm in which the cells are more 

 or less scattered in a homogeneous ground-substance or matrix, as 

 occurs, say, in the various connective tissues. The cells are stellate 

 and non-epithelial. 



Formation of the Embryo. 



Early Stages. 



The early stages in the formation of the embryo take place in 

 that part of the embryonic area which lies in front of the primitive 

 streak. These stages are three in number — ^namely, (i) the forma- 

 tion of the neural canal; {2) the formation of the notochord; and 

 (3) the formation of the mesodermic somites. 



Neural Canal. — ^The ectodermic cells of the head-process, in front 

 of the primitive streak, give rise to the neural plate, which very soon 

 presents a median longitudinal furrow, called the neural groove. 

 This groove is bounded on either side by a longitudinal ridge of ecto- 

 derm, these two ridges being known as the neural or medullary folds. 

 Anteriorly, these folds are continuous with each other in front of the 

 neural groove; but, posteriorly, they are separate, and embrace 

 between them the anterior end of the primitive streak and Hensen's 

 node, through which node the neurenteric canal passes. That 

 canal is therefore originally in direct communication with the 

 neural groove, subsequently the neural canal, its other communica- 

 tion being with the archenteron or primitive intestinal cavity. 



The neural folds, becoming elevated, incline towards each other, 

 and eventually they unite over the neural groove. In this manner 



